Radio frequency power, radial-flow, parallel plate plasma reactors are widely used in the semiconductor industry for both deposition and etching. For example, plasma reactors are widely used to deposit a thin layer of silicon nitride on a silicon wafer by exposing the wafer to a plasma in which the reactive species are silane and ammonia. The silicon nitride film which is deposited on the wafer is then used as a mask in subsequent processing of the wafer.
Unfortunately, in the example mentioned above, the ammonia is found to have a higher activation level than the silane; that is to say, it is more difficult to activate the ammonia than it is to activate the silane. As a result, a considerable degree of non-uniformity is observed in the radial distribution of the metastable, activated nitrogen which is obtained from the ammonia. This, in turn, leads to non-uniform physical properties and non-uniform stochiometry in the deposited silicon nitride. Of course, this phenonmenon is not limited to plasmas derived from ammonia and silane but is observed, in general, whenever two reactive species having differing activation levels are used in either plasma deposition or plasma etching processes.
A paper by Shibagaki, Horiike, Yamazaki and Kashiwagi, which was read before the Symposium of the Electro Chemical Society, held Oct. 9-14, 1977 in Atlanta, Ga., suggested the use of microwave energy to excite nitrogen gas which could then be combined with unexcited silane in a reaction chamber to deposit silicon nitride.
Independently, it occurred to me that microwave energy could also be used to pre-ionize one of the reactive species supplied to a radial-flow reaction chamber thus, in effect, allowing independent control over the activation of both species. Advantageously, the species that is pre-ionized is the species having the highest activation level; thus, eliminating the above-discussed problem of non-uniform distribution of the activated species in the plasma.